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Home , Daptocephalus, Endothiodon, Lystrosaurus

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provided by Wits Institutional Repository on DSPACE Palaeont. afr., 32, 75-,82 (1995)

DENTITION AND FEEDING NICHE OF (SYNAPSIDA; ANOMODONTIA).

by

E. M. L-atimer, C. E. Gow and B. S. Rubidge

BPI Palaeontology, University of the , Johannesburg, Private Bag 3, Wits 2050, South

ABSTRACT The cranial morphology and extensive dentition of the Endothiodon are different from those of any other dicynodont and it clearly had a different feeding niche. Recently prepared material from which has relatively well preserved dentition has revealed that the mode of tooth replacement of Endothiodon differs in the upper and lower jaws. The very high and narrow vaulted palate with a close fitting, pointed lower jaw indicates that Endothiodon had a small mouth with muscular cheeks and extensive cheek pouches, suggesting a browsing feeding niche. KEYWORDS: Therapsida, Dicynodontia, Endothiodon

INTRODUCTION MA TERIAL AND METHODS The Endothiodon is known from The Mozambiquean material consists of several , , Tanganyika (Cox 1964), fragmentary skull pieces, of which two snouts are the (Kutty 1972), and (Keyser 1981). However, most complete, and several incomplete jaws consisting because most of the skull material is fragmentary, of the symphyseal region and considerable portions of relatively little is known about the cranial anatomy of one or both tooth rows. All have suffered postmortem this atypical dicynodont. Several specimens collected damage, including total or partial loss of teeth, but in Mozambique in 1956 which are housed in the some crowns functional at time of death remain. As this collections of the Bernard Price Institute for is a surface collection which required no excavating, Palaeontological Research have recently been exposed bone has been weathered but freshly exposed prepared. Comparison of this material with specimens bone is very well preserved. from the of South Africa reveals Most preparation was undertaken mechanically important new information on the dentition and using an air scribe, but as the bone is not adversely possible mode of feeding of Endothiodon. affected by acid the carbonate rich matrix was speedily In South Africa, where the majority of Endothiodon removed in 5% formic acid, the bone being specimens have been found, the stratigraphic range of strengthened during preparation with a dilute, the genus extends from the Pristerognathus penetrating solution of Glyptal Cement. Assemblage Zone to the upper regions of the Cistecephalus Assemblage Zone (Smith & Keyser In DESCRIPTION Press a & b). Because its stratigraphic range is Lower Jaw relatively limited the genus has been used as a zone Although the purpose of this paper is to describe the fossil in previous biostratigaphic subdivisions of the dentition of Endothiodon, as the lower jaw of this Beaufort Group in South Africa (Broom 1906, 1907, genus has not been described in detail it is here 1909; von Huene 1925; Watson 1914 a,b). described briefly as far as preservation of the material The Mozambiquean material was sent to Dr S H permits. In the collection studied only the anterior Haughton, then director of the Bernard Price Institute, portions of the jaw rami are preserved and the in 1957, by Dr F Mouta, who had collected it in the surangular, prearticular and articular bones are Lugno area of Niassa Province in 1956. mlssmg. _ Cox (1964), in revising the of In the symphyseal region the dentaries are curved Endothiodon, used the palate and dentition to reduce upwards into a sharply pointed and narrow beak which nine to three. He also speculated that the fits closely into the highly vaulted palate of the upper may have fed by grubbing vegetable matter out of jaw. A broad groove is present on the dorsal surface of swampy ground. In contrast, our research suggests that the dentary lateral to the teeth. Endothiodon was a specialised browser. The genus is A lateral dentary shelf is absent in Endothiodon unique among in having premaxillary (Cluver & King 1983), but in all specimens examined teeth and also has differing modes of tooth replacement a bulbous swelling of the dentary is present on the in upper and lower jaws. ventrolateral side of the jaw (Fig. 1 A&D) anterior to 76

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\ \...... -"', . 'r'.... _J.,' \ . \ : \ . I I \.,--' Figure 1. Endothiodon Stereophotographs of lower jaws illustrating arrangement of teeth. Lines join members of the same Zahnreihe. B has some upper teeth in natural association in the matrix - these are clearly much thicker than the lowers. A = BP\1\'i494; B = BP\1\'i492; C = BP\1\'i489; D = BP\1\'i491 ; Scales are 50mm. the intramandibular fenestra. This swelling is not Cox(1964) considered that Endothiodon has three however the same as a lateral dentary shelf and the parallel rows of teeth in the dentary. However it is now function is not yet understood. apparent that it is not possible to group the teeth in The splenial is a relatively large bone and typically parallel longitudinal rows, rather they fall into obliquely constitutes most of the ventral surface of the arranged Zahnreihen (Edmund 1969) or replacement symphyseal region. The splenials form an waves (Figure 1). Zahnreihen are a series of consecutive anterodorsally oriented wedge between the dentaries, replacement waves; each Zahnreihe is a row of teeth in and in longitudinal section the splenial shows a distinct which the oldest is the anteriormost and the youngest undulation within the symphysis at the anterodorsal the posteriormost, i.e. teeth have erupted sequencially part of the suture with the dentary. Posteriorly the from front to back. Most amniotes have single rows of splenial tapers to a point, low on the medial surface of marginal teeth; such retention of several Zahnreihen is the dentary. an uncommon phenomenon.Teeth of more posterior The angular forms the largest portion of the ventral Zahnreihen tend to be smaller, as is commonly the case border of the lower jaw extending forwards as far as a in the marginal dentitions of amniotes generally. In all point in line with the posteroventral border of the cases where the dentitions are sufficiently well symphysis, where it forms a pointed contact between preserved (BP\1\5489 to 5494 and 5497), it can be seen the dentary on the lateral side and the splenial on the that new teeth were being added to even the most medial side. None of the fragments have the posterior posterior Zahnreihen, showing that there was active, region of the angular preserved. ongoing replacement throughout the tooth row......

77

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Because of the perslstmg Zahnreihen the tooth The crowns of unworn lower teeth are broad bearing portion of the dentary is a broad zone in which mesially, tapering towards the distal edge which bears the teeth are held in an area of spongy bone that is sharp, posteriorly directed serrations extending almost continually being remodelled. This area, which to the tip (Figure 2 B,C,E & F). Roots are cylindrical. broadens posteriorly from the posterodorsal margin of Only two undamaged crowns of teeth worn through use the symphysis, extends medially from the medial are present in the collection (Figure 2 A,B & C). These margin of the dentary to the lingual edge of the lower show that crowns became faceted on their lingual and jaw. The labial margin of this zone, where it is exposed labial sides. This could only be effected by contact with by loss of teeth, displays deep, concave impressions of food. In all cases where they are preserved, the rounded the roots (Figure 2D). The lingual margin (Figure 2D) tips of the teeth take on-a high polish (like hard objects bears a sequence of foramina within (behind) which tumbled in a vegetable fibre medium). The crown facet developing, unerupted teeth can be seen. The margins on the labial side of the second tooth bears fine of these crypts erode upwards as the new tooth grows horizontal striations which are visible under a light (Figure 2F); the alveolar border then closes as the tooth microscope and were most probably caused by abrasive moves labiad. As the majority of (especially fully particles in the food. functional) crowns are missing, it is not possible to work out the exact sequence of replacement. However Upper Jaw it is apparent that there was no bilateral symmetry to the Upper Dentition process (Figure 1). It is clear that replacement waves The teeth of the upper jaw, which are roughly moved posteriad along the jaw, as shown by the relative positioned in a single row, are considerably more maturity of new teeth. robust, having at least twice the cross-sectional area of 78

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Figure 2. Endothiodon BP\1\5497. A,B & C are labial, lingual and distal views respectively of two left lower teeth. D is a sketch of the same specimen to show empty sockets behind these teeth, and the crypts within which new replacement teeth develop. E and F are lingual views of lower teeth of BP\1\5292. Scale bar: 50 mm. the lowers, and occlude lateral to them (Figure 1 C). Lines in Figure 3 B indicate that there is a consistent Only in BP\1\5489 are upper crowns preserved, as in direction in which teeth migrate in the upper jaw, but i the case of the lowers , upper crowns are worn to essence there are single, rather irregular rows of teeth produce ligual and labial facets, and their tips have a in each maxilla. high polish. Cox (1964) reported that the upper teeth of Hopson has pointed out to us that there is a suggestion Endothiodon are serrated on their mesial edges. that the upper dentition, like the lower dentition, may The replacement pattern of the upper teeth is not as be interpreted in terms of Zahnreihen. We agree, easily determined as that of the lowers. Certainly it however, because we have mostly empty alveoli to deal differs from the usual reptilian pattern of alternate with we are not confident to propose what the pattern replacement. Possibly because they are more robust, might be. the upper teeth did not need replacement as frequently as the lowers. Specimen BP\1\338 (Figure 3 B), which FEEDING AND MASTICATORY FUNCTION is relatively well preserved, permitted some As Cox (1964) has suggested, upper teeth were understanding of the mode of replacement of the upper probably opposed by a horny pad occupying the dentary teeth. In this specimen, as in all the specimens studied sulcus labiad to the lower teeth (Figure 1); this is a where this part of the skull is preserved, the first two fairly laterally restricted area into which to have to bite, teeth are borne by the premaxilla. The first position is which could just accommodate the degree of lateral occupied by a mature tooth. In the second position are drift which occured during the life of the teeth. two confluent alveoli, in the more lingual of these the Lower teeth were not as restricted. As Cox(1964) tip of a replacement crown is just visible, while the pointed out these teeth occluded against the palatines, more lateral cavity represents a tooth recently shed. which presumably were also covered in life by horny The first tooth position in the maxilla bears a mature pads. The palatine pads would have been shorter than tooth about to be shed. Resting against its lingual the lower tooth rows: with the jaw fully occluded the surface is a new tooth in an advanced state of eruption. pads would lie above the back of the tooth row. Thus for The next two positions are occupied by the roots of the front of the tooth row to be effective, the jaw would mature teeth, the crowns of which are missing; labiad to have to be drawn back. This would also allow opposing these teeth are pits from which previous teeth had been patterns of upper and lower tooth serrations to effectively shed. The next two positions are represented by empty, slice up food in the mouth. This is excellent supporting ovate sockets; the next by a mature tooth with an empty evidence for the propalinal jaw movements envisaged socket labiad to it, and the last position by a newly for dicynodont anomodonts by Crompton and Hotton formed but badly damaged tooth. (1967) and King, Oelofsen & Rubidge (1989). 79

A

B

Figure 3. Endothiodon Stereophotographs of upper jaws, illustrating arrangement of teeth. Direction of movement of replacing teeth indicated by arrows. A. BP\1\5498 a small individual with a replacing second premaxillary tooth. B. BP\1\998. Right upper dentition. Lines indicate single tooth positions within the row. C. BP\1\5499. Upper dentition - interpretation only possible in the light of B. Scale bar: = lOmm 80

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Figure 4. Endothiodon A and C Stereophotographs of the large snout BP\I\'5505 (B is BPjlj5506). A. Left lateral view photographs and drawings same scale. B. Snout detail to show ventrolateral groove in premaxilla. C.Palatal view. Scale bar = 50 mm 81

Important to these considerations is the unique shape the edge of the upper jaw. Cluver(1971, p. 191) cites of the snout, palate, and tip of the lower jaw. The Ewer(1961) and Watson(1912) in support ofthe notion premaxillary part of the palate rises steeply (Figure 4A that and () and C) to accommodate the pointed, upturned tip of the were able to close the external nares by means of lower jaw. This vaulting reaches so high (Figure 4A) muscular flaps attached to vascularised pits in the that the floor of the nasal cavity slopes up to ventral narial border, involving premaxilla, maxilla accommodate it and the anterior tip of the upper jaw is and septomaxilla, but this is speculative. The groove in located just in front of the external nar~ This would Endothiodon is clearly quite different; three possible make mud grubbing very awkward. The symphyseal functions suggest themselves. region of the sharply pointed lower jaw bears a narrow, l.1t permitted the recycling of condensed, exhaled, shallow, dorsal groove, the purpose of which is unsure water vapour. 2. It may have been the tract for a scent­ (Figure lB). marking gland which deposited a pheromone (or a This forceps-like muzzle is a superb mechanism for cocktail of pheromones), either passively as the animal a specialised browser, which could rapidly nip off fed, or in a controlled way by rubbing the snout on individual which would be passed back to be suitable objects. 3. It might conceivably be the duct of comminuted by the cheek teeth prior to swallowing. a salt (sensu lato) gland, accummulated salt being A prominent premaxillary groove extends from the rubbed away as the animal fed. These possibilities ventral border of the external naris to the edge of the appear incompatible with a keratinous beak. The upper jaw, (Figure 4 A and B). This structure is unique animal probably had cheeks as has been proposed by among anomodonts and its possible function is Cluver (1975) for Chelydontops. One can thus discussed below. visualise a tiny mouth confined to the tip of the muzzle, used, like forceps, for precision cropping of foliage, but DISCUSSION an extensive oral cavity in which food was processed The dentition of Endothiodon differs from that in prior to swallowing. more typical reptiles, including the dicynodont genus These suggestions are at variance with those of Cox Emydops (Hopson 1964), in which replacement is rapid (1964, p.18), who suggested the may have and alternate. 'grubbed vegetable matter out of the ground, possibly The multiple rows of dentary teeth in Endothiodon in the extensive mud swamps of the period'. The are the result of retention of several members of each closeness ofthe nares to the tip ofthe snout and the total Zahnreihe, and it appears that tooth replacement lack of enlarged canines, or even caniniform processes, continued throughout the animal's life. Although are additional reasons why we find this implausible. Endothiodon is the only dicynodont to exhibit such a Modem perceptions of the palaeoecology of the dentition in the lower jaw, similar arrangements are lower Beaufort differ from that of Plum stead (1963) recorded in (for example) Captorhinus (Bolt and cited by Cox (1964). Smith (1993) regards the Beaufort DeMar 1975), and some procolophonids (Gow 1977). as consisting of predominantly fluvially deposited Replacement in the upper jaw was more usual, with rocks, with floodplain accretion rates ranging from never more than two teeth (of different ages) present at OA-5.Smm\yr, but with palaeosols suggesting periods each tooth position; essentially there was a single row of stasis from 5000 - 10000 yrs. Smith's taphonomic of functional upper teeth on each side. reconstruction (his Figure 21) shows well established, The probable nature of the integument in the jaw dense, riverine vegetation, rapidly giving way to arid region is an intriguing problem. It is interesting that conditions away from the rivers. Cox (1964) makes no mention of an extensive turtle­ We visualise Endothiodon as inhabiting the dense like horny beak as is usual for ; he confines riverine vegetation (as the most likely source of quality himself to proposing horny pads on the palatines and in browse), occupying a trophic niche not available to the lateral dentary groove. There are essentially two smaller, more fully terrestrial dicynodonts, and would reasons why it is difficult to conceive that the animals therefore predict that articulated Endothiodon had a full, turtle-like beak. One is the incredibly close skeletons, i.e. those which have fossilised in situ, fit between the bones of the upper and lower jaws: all would most likely be found close to fossilised major that can be accommodated here is tough skin (this river courses. would accord well with a browsing habit). The second problem is the presence of the groove in ACKNOWLEDGEMENTS the external surface of the premaxilla which runs from We thank Kevin Padian and James Hopson who refereed the a vascularised region at the ventral border of the naris to manuscript and offered many constructive suggestions. 82

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